An aromatic polyimide film has excellent various properties such as heat resistance, dimensional stability, electrical properties, and flame resistance, and also has flexibility, and therefore it has been widely used for a flexible printed circuit board (FPC), a TAB substrate, a COF substrate, and the like.
A multilayer FPC comprising a polyimide film is produced, for example, by laminating a polyimide film, which is to be an insulation layer, onto a wiring board having a copper wiring layer formed on a polyimide film, which is to be an insulation substrate, via a bonding sheet, and then forming a copper wiring layer on the surface thereof. A polyimide film having higher stiffness and lower coefficient of thermal expansion is preferred, because the polyimide film has higher handleability during processing/fabrication and a laminate in which a metal layer is laminated on the polyimide film has higher dimensional accuracy. On the other hand, the polyimide film tends to have lower adhesiveness to a metal layer, and therefore an adequate peel strength may not be achieved. Among various polyimides, a polyimide having high chemical stability and being strong, in particular, is hardly influenced by an extrinsic operation, and therefore has the problem of hardly having an adequate peel strength as compared with a chemically sensitive polyimide.
A surface of an insulation layer where a wiring layer is formed may need to be subjected to roughening treatment to achieve anchor effect which is a mechanical linkage, or other treatment to improve physical or chemical interaction so as to ensure the adhesion to the wiring layer (generally, metal layer) formed thereon. As a method for improving the adhesiveness of a polyimide film to a metal, a method wherein an aminosilane-based, epoxysilane-based, or titanate-based heat-resistant surface treatment agent is used during the film production is proposed (Patent Literature 1). Additionally, a method wherein a surface of a polyimide film is treated with an aqueous solution containing potassium permanganate and/or sodium permanganate, and potassium hydroxide and/or sodium hydroxide is also proposed (Patent Literature 2).
In addition, a polyimide film in which a layer of an amorphous polyimide containing a heat-resistant surface treatment agent is formed on at least one surface of a core layer formed of a polyimide having higher stiffness and lower coefficient of thermal expansion (Patent Literature 3) and a polyimide film for metallizing, which has a thin layer formed of a certain polyimide and containing a heat-resistant surface treatment agent on one surface or both surfaces of a polyimide layer (Patent Literature 4) are also proposed. Additionally, a laminate wherein a resin material consisting of “polymer film/plating formation layer containing crystalline thermoplastic resin” is plated, and the laminate consists of “polymer film/plating formation layer containing crystalline thermoplastic resin/plating layer” (Patent Literature 5) is also proposed.
A connection between wiring layers in a multilayer FPC is made by forming a via in a polyimide film, which is an insulation layer, and plating the inside of the via with metal. The formation of the via as described above is performed by laser, or the like. Therein a resin residue (smear) inevitably remains in the bottom of the via, and therefore a desmear treatment to remove the smear is indispensable. The desmear treatment is often performed by a method wherein etching is chemically performed with an alkaline aqueous solution of permanganate, or the like. By the desmear treatment, the surface of the polyimide film having adhesiveness improved as described above may be affected and thereby adequate adhesiveness may not be achieved, or the uniformity of the surface of the polyimide film may be impaired and thereby the electrical reliability may be decreased when a fine circuit is formed thereon.